Abstract: In this paper, the strength characteristics of the red clay under five temperature gradients ranging from 27 to 95 ℃ were studied by a strainstress controlled direct shear apparatus. The test results show that the temperature effect of the strength characteristic parameters is affected by the water content and the dry density, but the temperature sensitivity of the water content sample on the strength characteristic is obviously higher than that of drier density sample with less water content; when the temperature is lower than 65 ℃, the strength characteristic and temperature increase positively; Within the range from 50 to 65 ℃, the φ decreases with the increase of temperature, showing a state of thermal softening of the red clay; when the temperature is higher than 65 ℃, the shear strength increases in proportion to temperature, showing a state of shear hardening. Therefore, the critical temperature of thermal hardening and softening of the red clay is 65 ℃. The critical temperature of thermal hardening and softening of red clay is 65 ℃, which is valuable for the studying of the temperature effect on the strength characteristics of the insitu red clay.

Key words: cohesion, internal friction angle, temperature effect, direct shear test